824 _ On Aerial Navigation. 
proper movement when the machine was at anchor to the earth 
from some point under the prow. It ought likewise to be ob- 
served, that, owing to the internal pressure there will be exerted 
a power of about 460 pounds per lineal yard, endeavouring to teat 
the cloth asunder. This power is easily provided against by a 
wide netting, although most cloths would sustain it without such 
additional strength. Another circumstance to be adverted to, 
is the advantage that would arise from regulating the shape of 
ithe prow in the best manner for obviating the resistance of the 
air. Theory unfortunately is of little use upon this occasion, for 
Nature, always true to itself, makes the prow of the bird con- 
eave, and that of the fish convex to their axes; whereas theory 
would appoint both alike. In the absence of all good theory, 
I shall, as proposed in one of my former papers, give the form 
of the woodcock from actual measurement, that bird having 
frequently to cross at least 500 miles of sea at one flight, and 
Nature seeming to have united every contrivance to blend strength 
with lightness in its structure ;—hence it is more than probable, 
that as the resistance of the air was the obstacle which all this 
contrivance was calculated to overcome, the external form is 
nearly the best possible, being that which would more than all 
the rest tend to the ease of the performance. 
Fig. 1, Plate [V. shows a section of this bird through its axis. 
Fig. 2, is a tranverse section with the apparatus suspended. The 
ordinates are given in hundredth parts of an inch at the distance 
of one inch from each other. The weight of the individual bird 
corresponding with these measures was 124 ounces, and the ve- 
locity of its leisure flight in calm air will have been about 28 miles 
per hour. It may possibly require a different form of prow for 
obviating resistance best at different velocities, and what may be 
the best form for a small object may not be so, even at the same 
velocity, for a large one; but notwithstanding this, I know no 
better guide, and shall therefore recommend the experiment to. 
be made in the form of the prow delineated, as far as it can be 
made to agree with the flattened structure necessary in this in- 
stance for the trial of the inclined plane. That the practica- 
bility of constructing this balloon may be better judged of, I here 
subjoin the following estimates respecting it. 
The quantity of cloth and the general appearance seem 
enormous ; yet it must be recollected that it is only an inflated 
bag, and that the 5750 square yards it consists of would all 
pack up in a cubical bore 84 yards each way, allowing 1-5th of 
an inch for the thickness of each fold, which must be amply 
sufficient. The cloth 1 made use of in my experiments generally 
weighed half a pound per square yard ;_ but probably, including 
the netting, the estimate for the present purpose should be taken 
at 
ee 
